The invention relates to the properties of crystals, more particularly to the strengthening of crystals, especially for use in lasers.
Single crystals are used in a variety of applications such as in the electronics industry and in the optics industry. In the optical arts, single crystals are grown for laser media, laser amplifiers, harmonic conversion and other uses. Well known examples of laser media single crystals are yttrium aluminum garnet (YAG), gadolinium gallium garnet (GGG), and gadolinium scandium gallium garnet (GSGG).
Much of the work in lasers today is in further development of high average power lasers. Such lasers must not only have high energy in a given laser pulse, but must also have a high repetition rate of those pulses. In the past, laser media were predominately in two geometries: the rod for smaller sizes and the disk for larger sizes. Rods could be crystalline or glass, and larger sizes were made of glass. Due to its low thermal conductivity, glass cannot rapidly remove the heat generated during laser operation. Also, most crystals are inherently stronger than glass and have other desirable spectroscopic properties. Thus, crystals are the favored material for the new laser media.
To minimize adverse optical distortions and maximize surface area available for cooling, slab type geometries are superior to the older rod geometries for high average power solid state layers. The slab has large surface area to take off heat from the sides of the amplifier medium not in the laser beam, and at the same time can be formed from crystalline materials. Current designs for high average power slab lasers have rectangular slab geometries with dimensions on the order of 1 by 10 by 20 centimeters in a single crystal garnet as shown in FIG. 1.
The use of single crystal laser media in high average power laser applications permit superior beam quality and high power output. A fundamental limitation on power output is the component strength as well as thermal conductivity. High power elements require high tensile strength and good component durability. Most larger size prior art solid-state laser media were made typically of glass, and smaller size components were made in a rod geometry of YAG or glass. When the strength problem was encountered in glass laser media one method of increasing strength and resistance to abrasion was found to be placing a compressive, ion-beam sputtered film on the glass substrate as reported in J. E. Marion, "Development of High Strength Solid State Laser Materials", UCRL-93160 Abst. Summary, to be published in Advances in Laser Science, 1, Editors W. C. Stwalley and M. Lapp , Amer. Inst. of Phys. Proceedings, 146, pp 234-237 (1986). Attempts to extend compressive layer technology to single crystals have used ion bombardment as reported in T. Hioki et al, "Strengthening of Al.sub.2 O.sub.3 by Ion Implantation", Journal of Materials Science Letters, 3, pp. 1099-1101 (1984).
Because of the higher average powers at which the new crystalline laser media are expected to operate the laser media are placed under greater operating stress than ever before. These stresses arise from the temperature gradient in the slab. Absorption of flashlamp or other pumping energy leads to heating of the slab bulk. The active cooling of slab surfaces by high velocity fluids leads to a steady state thermal gradient within the slab. The thermal gradient gives rise to biaxial tensile stresses at the slab surfaces whose magnitude can approach or exceed the component strength.
Thus, strengthening a single crystal laser medium permits higher average power output. One approach to strengthening has been to remove or minimize subsurface damage. Subsurface damage has been shown to be removed by the methods of acid etching and by large amounts of material removal in the grinding and polishing fabrication steps. This method is reported in J. E. Marion, "Strengthening of Solid State Laser Materials", Appl. Phys. Lett., 47, pp 694-696 (1985) and is responsible for up to 15 times increase in crystal strength. However, it has been difficult to fully implement this method because it has not been possible to preserve the pristine surfaces throughout the entire process of handling, mounting and use in the laser.
Thus, there is a long standing, unfulfilled need for the strengthening of single crystal laser media. If this need is met, then lasers of significantly higher average power are possible.